(a) Field of the invention
The present invention relates to an imaging lens system which uses at least one graded refractive index lens element of the so-called radial type having refractive index distribution in the direction perpendicular to the optical axis (radial GRIN lens element).
(b) Description of the prior art
As imaging lens systems consisting of small numbers of lens elements out of the conventional imaging lens systems, there are known triplet type and Tessar type of lens systems. Each of the lens systems of the triplet type or Tessar type consists of three or four lens elements. Speaking more concretely, the imaging lens system of this type is composed of a positive lens element, a negative lens element and a positive lens element, or a positive lens element, a negative lens element, a negative lens element and a positive lens element, and the object side surface of the second lens element having the negative refractive power is generally designed as a concave surface. This concave surface is adopted for correcting negative spherical aberration in the imaging lens system as a whole, but aggravates coma and curvature of sagittal image surface, thereby degrading optical performance of the imaging lens system. Accordingly, this type imaging lens system cannot have a high aperture ratio while correcting aberrations favorably over the entire range of the image surface and is used with lens shutter cameras, but scarcely adopted for use with single-lens reflex cameras for which higher optical performance is desired.
Japanese Patent Kokai Publication No. Sho 55-65918 disclosed a Tessar type imaging lens system which is designed so as to have a high aperture ratio of F/1.7 by using an aspherical surface. In this lens system also, the object side surface of the second lens element is designed as a concave surface and coma is not corrected sufficiently favorably.
Japanese Patent Kokai Publication No. Sho 58-142309 disclosed another imaging lens system of the Tessar type in which the object side surface of the second lens element is designed as a convex surface and coma is corrected favorably. However, this lens system has an aperture ratio of F/2.8 due to insufficiently corrected spherical aberration and cannot be said as a lens system which has a sufficiently high aperture ratio.
Further, Applied Optics Vol. 25, No. 18 showed design examples of lens systems using graded refractive index lens elements. However, this literature clarified no difference between the design examples and lens systems composed only of homogeneous lens elements or made no attempt to enhance the aperture ratio of F/2.8 of the lens systems described therein.